Generally, in gear devices such as transmissions (gear boxes) and differential gears (differential arrangements), mechanisms are lubricated and cooled down by being supplied with gear oil (lubricating oil). The lubrication with the gear oil provides the effect of forming oil films over the mechanisms, which reduces friction between metal pieces and allows smooth movement. The cooling provides the effect of absorbing heat produced by friction and the like and transmitting that heat to the outside.
However, the gear oil is high in viscosity in order to maintain itself as an oil film. This viscosity has such a property that the higher the temperature, the lower the viscosity becomes. Thus, the gear oil at a low temperature has such high viscosity that a vehicle has a drive loss due to an increase in friction (frictional resistance). This has been one of the causes that deteriorates fuel efficiency.
On the other hand, reduction in fuel consumption of vehicles has been advanced rapidly in view of energy saving. Here, in achieving energy saving, it is an important issue to reduce the friction caused by the gear oil at a low temperature to improve the fuel efficiency of a vehicle.
Conventionally, warm-up of a gear device such as a transmission (gear box) or a differential gear (differential arrangement) has been done naturally by temperature raise through friction between gears' tooth surfaces, a lubricating oil, churning energy of the lubricating oil, or transmission of heat from an engine (internal combustion engine). Thus, the gear device includes no exclusive device to accelerate the warm-up. Moreover, the gear device is filled with a specified amount of oil inside its housing (enclosure), and the lubrication and the cooling are done with components dipped in the oil. Thus, a drive loss resulting from the churning resistance is increased. As a result, the fuel efficiency is deteriorated.
The viscous resistance of the lubricating oil cannot be ignored particularly in the case of cold start of the engine from a state where the temperature of cooling water and the temperature of the lubricating oil are low. In terms of the amount of fuel injected during idle, for example, the cold start requires more fuel than hot start in order to make the engine idle speed stable at a preset value. Since transmissions and differential gears are distant from their engines which are heat sources, it takes time to raise the temperature of the oil, and hence the driving resistance cannot be easily reduced. Accordingly, the fuel efficiency is deteriorated. Such an influence is remarkable when the ambient temperature is low. Thus, the temperature of the lubricating oil in transmissions needs to be raised quickly.
To this end, there is a device which introduces oil into a heat exchanger where the oil is heated by cooling water immediately after an outlet in an engine when the temperature of the oil in the transmission is low, whereas the oil is cooled down by the cooling water before being heated by the engine when the temperature of the oil is high (see Patent Document 1, for example). However, only heating with a heat exchanger cannot provide a solution for quickly raising the temperature of the oil especially at cold start.
Meanwhile, there is an oil temperature control device which includes a heat exchanger for heat exchange of lubricating oil, an oil temperature sensor that measures the temperature of the oil in the transmission, and an oil control valve that controls the flow rate of the lubricating oil, and which controls the temperature of the oil by controlling the amount of the oil into the heat exchanger with the oil control valve (see Patent Document 2, for example). However, this device is a control device for a situation where the temperature of the oil has become high, and is not a control device for proactively raising the temperature of the oil. Thus, the device cannot reduce friction caused by the lubricating oil at a low temperature.